This invention relates to an air/fuel ratio feedback control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to an arrangement which is capable of monitoring with accuracy the position of an actuator for driving an air/fuel ratio control valve to thereby ensure achievement of highly accurate air/fuel ratio control.
An air/fuel ratio feedback control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, has already been proposed by the assignee of the present application, which comprises means for detecting the concentration of an exhaust gas ingredient emitted from the engine, fuel quantity adjusting means for producing the mixture being supplied to the engine, and means operatively connecting the concentration detecting means with the fuel quantity adjusting means in a manner effecting feedback control operation to control the air/fuel ratio of the mixture to a predetermined value in response to an output signal produced by the concentration detecting means, the connecting means including an electrical circuit, an air/fuel ratio control valve, and a pulse motor arranged to be controlled by the electrical circuit to drive the air/fuel ratio control valve.
In this proposed air/fuel ratio feedback control system, a reed switch is provided which is adapted to be opened or closed when a pulse motor, which is used as an actuator for driving an air/fuel ratio control valve, passes a predetermined position set as a reference position for the pulse motor. This predetermined position, which thus corresponds to the switching point of the reed switch, is previously stored in a memory in the electrical circuit of the connecting means. At the start of the engine, reaching of the pulse motor to the above predetermined reference position during driving of the same is detected on the basis of turning-on or -off of the reed switch to determine the initial position of the pulse motor. Air/fuel ratio control operation following the start of the engine is carried out with the value stored in the memory in the electrical circuit regarded as the reference position of the pulse motor.
According to this proposed air/fuel ratio feedback control system arranged above, the pulse motor per se does not produce any signal indicative of its actual position. Instead, the actual position of the pulse motor is monitored in an indirect manner such that the count in the actual position counter which counts pulses supplied from means in the electrical circuit for supplying driving pulses to the pulse motor is regarded as the actual position of the pulse motor. Thus, the actual position of the pulse motor is not directly monitored.
However, there can occur a disagreement between the count of the actual position counter and the actual position of the pulse motor due to skipping or racing of the pulse motor. As a consequence, it is impossible to control the air/fuel ratio of a mixture being supplied to the engine to proper values during open loop control where the actual position of the pulse motor has to be exactly known by the electrical circuit.